This invention relates generally to image forming devices and methods having image quality diagnostics. More particularly, this invention relates to electrophotographic image-forming devices and methods having image quality assessment by on-line measurement of toner density.
Electrophotographic image-forming devices are used to transfer images onto paper or other medium. Generally, a photoconductor is selectively charged and optically exposed to form an electrostatic latent image on the surface. Toner or other developing material is deposited onto the photoconductor surface. The toner is charged, thus adhering to the photoconductor surface in areas corresponding to the electrostatic latent image. The toner image is transferred to paper or other medium. The paper usually is heated for the toner to fuse to the paper. The photoconductor is then refreshedxe2x80x94cleaned to remove any residual toner and chargexe2x80x94to make it ready for another image.
In many electrophotographic image-forming devices, an output copy of a test reference is visually inspected to evaluate the image formation and development process. The test reference is essentially a xe2x80x9cperfectxe2x80x9d image of the desired output of the electrophotographic image-forming device. A service technician makes a copy of the photographic test reference and compares the copy to the test reference. If image quality is unacceptable, the toner density is adjusted.
The toner density amongst other factors affects the output image quality. Mechanical damage to any of the electrophotographic subsystems may introduce artifacts. The lifetime of consumables may degrade performance. Material fatigue may affect the spatial distribution of the toner (the image may have the correct toner density, but not be in focus).
The test reference approach provides a qualitative assessment of the image quality, which provides an inferential evaluation of the toner density on the photoconductor. The toner density is adjusted to adjust the image quality. While the image quality may visibly appear to be fine, service, maintenance and other problems may not be readily visible especially in the early stages. Generally, such problems have to become xe2x80x9cvisiblexe2x80x9d before a user or service technician knows the problem exists.
In addition, the test reference approach also is not well suited for performing maintenance and service routines. Some variations in toner density, while sufficient to for proper maintenance and service, may not be visible. Due to its qualitative nature, the test reference approach has varying results depending on the service technician""s experience level, training, and other subjective factors. As a result, the test reference approach causes maintenance and service routines to be performed prior to or later than the time they are needed. Also, service problems and maintenance are not early identified. Consequently, the image-forming device experiences unexpected downtime and additional repair and maintenance costs.
The use of greyscale tone reproduction magnifies the difficulties of using this test reference approach to assess the image formation and development process. In greyscale tone reproduction, the toner is applied in variable amounts from no toner to maximum toner to create an image. The density of the toner determines whether a portion of the image is black, white, or one of various shades of grey. These variations in toner density make the quantitative evaluation of greyscale electrophotographic processes by visual inspection virtually impossible.
To avoid the difficulties associated with visual inspections of the test reference, commercially available lightness meters and densitometers provide off-line visual evaluation and measurement of the printed density. However, these techniques require additional equipment and additional set-up time to use the equipment. They tend to extend or delay service calls and also are inferential evaluations of the toner densities on the photoconductor.
Accordingly, there is a need to provide an on-line, quantitative assessment of the image quality in an electrophotographic image-forming device.
The present invention provides an electrophotographic (EP) image-forming device and related method for on-line quality assessment of the image formation and development process. The EP image-forming device and method quantitatively assess the density of the toner applied to the photoconductor.
A toner density map of the entire photoconductor is provided in one aspect of the present invention. The EP image-forming device has a reference system for locating positions on the surface of the photoconductor. The reference system uses a reference point on the photoconductor and a sequencer to determine locations from the reference point along the length or circumference of the photoconductor.
The reference system identifies the locations of density measurements. For every density reading taken, there is a reference position. In this manner, the density readings are taken and evaluated at the same location. The measured toner densities for different locations may be assembled to provide a density map of the toner on the photoconductor.
The EP image-forming device has a photoconductor mounted to rotate on support rollers. A primary charger, an exposure device, a toning station, a transfer charger, a fusing station, and a cleaner are operatively disposed about the photoconductor. A transmission densitometer has a light emitter and a light collector, which are operatively disposed adjacent to the photoconductor. The densitometer also is connected to a microprocessor having a memory. The microprocessor in turn is connected to input and output interfaces. While single components are illustrated, there may be multiples of any components including the densitometer.
In the EP image-forming device of the present invention, an image frame on the photoconductor is charged. A step tablet or test image is optically exposed to form an electrostatic image on the image frame. Preferably, the step table is for greyscale tone reproduction. The electrostatic image has step areas corresponding to the steps of the step tablet. The toner station is disabled so there is no toner application at this time.
The photoconductor density is determined for each step area of the step tablet on the image frame. Preferably, five or more photoconductor density measurements are taken of each step area by the transmission densitometer.
The densitometer measures a voltage reading corresponding to the amount of light energy passing through the photoconductor on an optical path between the light emitter and light collector. The voltage reading corresponds to the density of the toner. The photoconductor voltage readings are stored separately in memory, with each photoconductor voltage reading identified as to its location on the photoconductor.
The image frame is charged for a second time. The transfer station and fusing device are momentarily disabled so the photoconductor passes without any interaction. The cleaner removes any charge from the photoconductor.
The step tablet is optically exposed to form an electrostatic image on the image frame for a second time. The electrostatic image has step areas corresponding to the steps of the step tablet. These step areas are the same as when the photoconductor voltage readings were taken.
The toner station deposits toner on the image frame. The toner forms a toner image corresponding to the electrostatic image, which corresponds to the step tablet.
The combined photoconductor and toner density is determined for each step area of the step tablet on the image frame. Again, preferably, five density measurements are taken of each step area by the transmission densitometer. These combined voltage readings also are stored separately in memory, with each combined voltage reading again identified as to its location on the photoconductor.
The average measured toner density is ascertained for each step area of the image frame. For each step area, the photoconductor voltage reading is subtracted from the combined voltage reading at the same location to provide a measured toner voltage reading or density for that location. The measured toner voltage readings are averaged for each step in the step tablet to provide an average measured toner voltage reading for each step.
The average measured toner voltage readings are compared to the aim toner voltage readings for each step in the step tablet. The aim toner voltage readings are according to the manufacturer""s specifications for the EP image-forming device.
The measured toner voltage readings are indicative of the toner density. Too high a measured toner voltage reading means there is too much toner. Conversely, too low a measured toner voltage reading means there is too little toner being applied. In either case, the toner application may be adjusted and retested until the measured toner voltage reading equals or is within an acceptable range of the aim toner voltage.
Preferably, every image frame on the photoconductor is assessed successively. If each the image frame on the photoconductor is assessed, then the average measured toner voltage reading is representative of the step on the step tablet for an entire revolution of the photoconductor.
The present invention quantitatively assesses the image quality of an EP image-forming device, using a reference system to provide a toner density map of the toner on the surface of the photoconductor. Actual or measured densities of toner are compared with aim densities according to the manufacture""s specifications and independent of the operator""s subjective visual inspections and comparisons. The densities of the toner are indicative of the image quality in an EP process.